1. Field of the Invention
This invention relates generally to methods for the pretreatment of microbial sludges to enhance subsequent biodegradation. More specifically, this invention relates to techniques for pretreating microbial sludges using sonication and shear forces.
2. Description of the Prior Art
Modern municipal sewage waste treatment plants utilize conventional mechanical and biological processes to reclaim wastewaters in a process which has an overall effect of converting a water pollution problem into a solid waste disposal problem (sludges). In a typical modern treatment plant the large objects and the grit are separated. Then the wastewater goes to the primary sedimentation tanks which remove 50%-70% of the suspended solids and 25%-40% of the BOD.sub.5. This sludge and the ground screenings are sent to anaerobic digestion. The wastewater then flows to the aeration tanks, where the colloidal and dissolved organic matter are converted into gases (primarily CO.sub.2) and cell mass by the aerobic growth of microorganisms, principally bacteria. The cell mass is then removed in the secondary sedimentation step and sent to anaerobic digestion. Increasingly, a final biological step is used to reduce the nitrogen content of the effluent by converting the ammonia to nitrate and then to nitrogen gas. This process also produces a biological sludge. The combined sludges are thickened to increase their solids content and sent to anaerobic digestion. The primary purposes of digestion are to reduce the organic content, volume, and odor potential of the sludge, and to reduce the concentration of pathogenic microorganisms (Metcalf & Eddy 1979, U.S. EPA 1979, Arora 1980, Federal Register 1989). The dewatered sludge from anaerobic digestion is the largest solid output from sewage treatment plants and presents the major disposal problem.
The disposal of microbial sludge solids resulting from conventional municipal sewage treatment has historically been expensive because of the extremely large volumes with which these sludges are produced. These sludges contain high fractions of volatile solids (VS), they retain large amounts of water (70-85% before drying), and because of the substantial bulk of the waste, transportation and disposal costs are significant. Recently, the costs for disposal of these microbial sludges through conventional landfilling has risen dramatically because of decreasing landfill availability. In some areas microbial sludges are banned altogether from the landfill because of the high pollution potential which these materials represent.
In light of the increasing cost for disposal of sewage derived microbial sludges, a substantial amount of work has been expended into alternatives to landfill disposal such as technology which may further reduce the water content of sludges in order to reduce the bulk of the waste requiring disposal. Public concern over possible hazardous emissions through combustion processes and possible heavy metal contamination from the resulting ash has reduced acceptance of combustion (Samela, et al. Environmental Aspects of the Combustion of Sewage Sludge in a Utility Boiler, Environ. Progress, 5:110, 1986) as a disposal option for municipal sewage sludges. The land application of sewage sludge is also problematic as biological activity produces methane and residual volatile solids result in organics contaminating groundwaters.
The cost of disposing of a given amount of sludge is often high and is growing higher. Further, increased loads on existing treatment plants also lead to sharply higher disposal costs. Increasing environmental requirements on the quality of wastewater treatment have resulted in a more complex process which produces greater microbial biomass for disposal. See Laughton, P. J., "Upgrading a Water Pollution Control Plant to Meet Stringent Effluent Discharge Requirements", Water and Pollution Control, 117:14 (1979). The greater organic loading of wastewater streams has created a higher stress on the treatment process that often reduces the organic removal efficiency. See Mungsgaard et al., "Flow and Load Variations at Wastewater Treatment Plants", J. Water Pollution Control Fed., 52:2131 (1980). This reduced efficiency degrades the sludge's dewatering properties, substantially increasing the water content and volume of the waste. See Rutherford et al., "Realities of Sludge Dewatering", Proceedings of the National Conference on Municipal Treatment Plant Sludge Management, Orlando, Fla. (1986). Finally, reduced dewatering efficiency requires increased use of organic polymers to facilitate dewatering. See Novak et al., "Mixing Intensity and Polymer Performance in Sludge Dewatering", J. Environ. Engineer, 114:1 (1988); Bandak et al., "Polymer Performance in Sludge Conditioning", Proceedings of the Eighteenth Mid-Atlantic Industrial Waste Conference, Lancaster, Pa. (1986); Doyle et al., "Sludge Conditioning With Organic Polyelectrolytes", Proceed. of the Nat'l. Conf. on Municipal Treatment Plant Sludge Management, Orlando, Fla. (1986). Increased polymer usage increases both the disposal costs and the organic loading of the waste stream. The net result is that the amount and cost of sludge disposal can increase disproportionally when an existing plant must deal with increased loadings and clean-up requirements.
Most recent research has focused on reducing the volume of waste by improved dewatering. See Knocke et al., "Effect of Mean Cell Residence Time and Particle Size Distribution on Activated Sludge Vacuum Dewatering Characteristics", J. Water Pollution Control Fed., 58:1118 (1986); Barraclough et al., "Start-Up Optimization of the Mechanical and Chemical Parameters Influencing the Dewatering Performance of a Gravity Belt Filter Press Operation", Proceedings of the Eighteenth Mid-Atlantic Industrial Waste Conference, Blacksburg, Va. (1986); Katsiris et al., "Bound Water Content of Biological Sludge in Relation to Filtration and Dewatering", Water Res. 21:1319 (1987); Harries et al., "Design and Application of a Modern Solid/Liquid Separation Plant", S. African Mech. Engin., 37:481 (1987); Cobb et al. "Optimizing Belt Press Performance at Smurfit Newsprint", Tappi Proceedings--1987 Environmental Conference, Portland, Oreg. However, the pollution potential of the sludge is unchanged since the dewatering does not reduce the VS fraction.
The volatile solids content of sludges that have already been anaerobically digested range from 40%-75% and the content of undigested sludge is even higher. See Downing et al. "Used-Water Treatment Today and Tomorrow", Ecological Aspects of Used-Water Treatment, Vol. 2, C. R. Curds and H. A. Hawkes, Eds. (1983); Ramalho, Intro. to Wastewater Treatment Processes, 2nd ed. N.Y.: Academic Press (1983). Clearly, the potential for further reductions in sludge volume still exists.
There has not heretofore been provided effective and efficient techniques for pretreating microbial sludges to disrupt the macro-structure of the sludge, break cells open, and enhance subsequent biodegradation.